The Dallas daily herald. [volume] (Dallas, Tex.) 1873-1887, 1887-05-05
IRON GENERALIZATION
n eau ses Changes I'm the Fiber OF
Metal and Wcakens It-Hence Brok-
en- Bridges.

Since a passenger train on the
North British railway plunged from the
great bridge over the Firth of Tay into
the dark waters below, one stormy
December night seven years ago, car.
rying every living soul to certain death,
there has been no railroad disaster that
has excited so much public attention as
hat which recently occurred on the
Boston and Providence railroad at the
Bussey bridge neal Boston. In both
the cause appears TO have been a struc-
tural defect. TINS may also be said of
the Ashtabula accident, whose horrors,
however, were increased by the fierce
work of flames and the inclemencies of
4 bitterly cold winter night.
Bridge accidents are considered by
railroad men as incomparably the
worst to which travel by rail is expos
ed, and rs among the most difficult to
guard against absolutely. One source
Of danger was shown when the peculiar
effects of Il locomotives hammer blows
upon n track were explained. The
theory was advanced that a girder of
the bridge had borken beneath the en-
gine's drivers, whose pounding upon
the effectively constructed bridge was
thus primarily the cause of the accident.
This brings up a phase of railroad and
bridge accidents which is at present re-
ceiving n great deal of attention. Crys-
tallizatiom in the material of iron bridges
is an clc ment of danger now so well
recognized that the Pennsylvania Rail
road company has adopted the definite
policy of substituting stone for iron
bridges, and the same plan is being fol-
lowed by other railroad companies.
The lifetime of mn iron bridge is from
twenty TO twenty-five years, and con-
slant care is necessary to insure its
safety. The Ashtabula bridge was
thirteen years old. and at the time it
fell beneath the weight OF two engines
and n heavy express train the theory
was commonly advanced that crystal
liaation had impaired its original
strength. The two great moving
causes of crystal cation are vibration
and the hammer blows of the locomo-
tive, which gradually, by repeated i'm
paci, turn the fiber of iron or steel into n
mass of crystals, needing only some
unusual pressure or blow to cause them
to break apart. Add to this the oscil-
lation caused by the passage of trains
AT Il high rate of speed, and it will be
seen that an iron bridge is exposed TO
peculiar perils, which can only be
guarded against b. rigid inspection and
constant renewals.
Capitalization was one of the several
theories advanced by Park Benjamin, q
New York engineer, to account for the
Tay bridge accident. He said:
General indications 90 TO show that
he cause must be sought in an Abnormal
condition of the structure, or rather one
which did not enter into the calcula-
lions of the builders. Such a condition
would be the deterioration of the. metal
from its change from Il iibrous to a
crystalline state under repeated vibra-
lion. That this occurs in railway
bridges has been vigorously disputed
by many well-known engineers. On
he other hand, many examples IRE
luoted by different authorities to sup
BORT the theory. Appropos to this par
ocular accident, a distinguished French
engineer and iron founder now in this
country informes us that he has known
oars of iron made by himself from
scotch pig to change from tough iibrous
to n brittle crystalline structure in
travelling by rail only from the north
Of France 10 Paris. This is, Of course,
an extreme instance.
again, recent research has demon
strated that because n structure with
stands a large quiescent load that fact
is little proof of stability under repeat
ed shocks and vibrations. Metals are
believed to have a %''life." A bar, for
example, may stand a million vibra-
tons and break down ut the million
and first, and yet the last shock may
be lighter than preceding ones. At
tempts, however, to reduce this law to
practical application have elected an
| abundance of conflicting evidence; but,
nevertheless, il is well settled that in
no department of mechanics is an ex-
tended course of actual experimenting
more urgently needed or OF graver pnb-
lie Importance
The strength of the Tay bridge at the
time of its building was such that it
was pronounced perhaps the most re.
markable structure in the world.
Engineering. a British magazine speak
inc OF its test. when five engines weigh
ing $50 tons, were placed on a single
span. said: THE result is the COM
plete establishment of this fact $50 i'm
portant to the public, that the bridge
is strong out Of all proportion to its